No Hot Water Puzzle???

4 May 2012
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United Kingdom
Standard indirect vented hot water & heating system, Potterton/
glowworm boiler with 2-port motorised valves to rads & Hot Water
cyclinder. Fully pumped (1 pump serves both circuits)

I've only got lukewarm water coming from hot water taps & shower. (say
12 degrees Celsius).
The flow rate is normal.
Boiler on for 1 hour plus. Normally 30 mins sufficient.
Radiators hot (when central heating on)
Hot water motorised valve on (manual overide)
Cylinder coil flow & return pipes both hot.
Cylinder hot water draw off pipe coming out of cylinder top just
doesnt get hot, barely warm after even hour.

If there was air lock in cylinder coil then both coil flow pipes
wouldnt be hot.
If air lock in the hot water supply pipework including cylinder then
flow rate from taps wouldnt be normal.
If limescale around cylinder coil, it worked fine one day, then not
the next. Surely limescale buildup & effects would be noticed
If sludge in coil then why coil flow & return both hot.

Going to crawl up into loft tonight but cant at the moment think
anything up there would cause this like stuck ball valves.

Are my assumptions wrong or what am I missing?

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Blockage /air lock at the cylinder.
No circulation going on.
If you turn on the immersion I bet you get hot water. So the only possible reason is not water is flowing through the coil of the cylinder.
12 ° is lukewarm you? :eek:

If heat is going through the coil, and hot water is still cold, then the coil is scaled externally or sludged internally.
Had any new taps fitted or washing machine plumbed in, is your shower (if you have one) cold water mains and gravity hot water ? just thinking you may have cold water backing up into the cylinder keeping it cool, see if you have a running overflow from the tank in the loft.
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